The invention is directed to the use of a thin, pliable film used for sealing the cell(s) of a microplate. More specifically, the invention is directed to an economical, simple and effective means of sealing a vacuum dawn or pressure forced microfilter plate which seals the cells thereof individually, thus allowing the filtration in each cell to proceed undisturbed by the status of filtration in other cells. The subject sealing means or system ensures that the vacuum or pressure level above or below the plate is not affected by completion of the filtration in one or more cells while other cells remain in the filtering status.
Microfiltration plates are fitted with membrane or depth filters and are intended to allow for the micro or ultrafiltration of multiple samples simultaneously. Generally, a vacuum is pulled under the plate, or pressure is applied over the plate, as a driving force for the filtration to be completed. An inherent problem with these devices is that not all wells within a single microplate, which may contain as many as 96 or more wells, filter at the same rate. This difference in filtration rate among wells of the same plate may be due to various parameters of the filtration set-up, such as the use of a defective membrane or other filter media in a particular well(s), a defective seal on the well(s), sample variances, and other differences in the individual wells or samples. Once all of the sample liquid has been removed from a well, air replacing the liquid and flowing through the filtration well alters the vacuum or pressure driving force retarding the rate of filtration in the remaining wells which continue to function or filter. Air breakthrough may also cause turbulence below the filter, enhancing the possibility of splashing and cross-contamination of liquid in the receiver reservoir(s).
Attempts to solve the foregoing problem have been varied. Many involve the use of different seal designs. Among them are the use of perforated polyester or cellophane tapes, and the use of a rigid or semi-rigid cap which fits the top of the plate. The main drawback to these types of seals is the rigidity thereof.
U.S. Pat. Nos. 5,464,541 and 5,462,878 describe prior art rigid seals. These devices prevent air breakthrough. However, they create another problem. As the liquid filters under the seals, a vacuum is created above the liquid. Every time the air gap above the filtering liquid doubles, the pressure driving force for filtration halves, causing an ever decreasing filtration rate. This effect is dramatically magnified as the mount of liquid placed in each well is increased. An alternative to a rigid cap seal on the microfiltration device is to individually seal each well as the liquid volume approaches the filter, a scenario which requires constant attention.
Therefore, if no sealing device is used, filtration efficiency of the entire microplate decreases rapidly once filtration of a first well or cell is completed and air breakthrough occurs. If a rigid sealing device is used to prevent air breakthrough, filtration efficiency of each well decreases at different rates, depending on the amount of liquid originally placed in each well and its filtration rate.
Other attempts to correct the seal problem described above include the use of a compliant, reusable sheet which does not seal the wells, but rather creates a bubble-point as all of the sample is drawn from a well, stopping filtration in that well. In this device, embodied in U.S. Pat. No. 4,927,604, no seal is created within the meaning of the current invention.
The use of a pliable polymeric film for sealing microplates solves both the air breakthrough and decreasing filtration rate problems discussed above.
It is therefore an object of the subject invention to provide a means for sealing a microfiltration device such that filtration of all wells therein proceeds to completion effectively and efficiently.
It is yet another object of the invention to provide a means for sealing individual wells within a multi-well filtration plate, as each well empties, so as not to disturb the filtration process taking place in the remaining wells.
It is another object of the invention to use a pliable polymeric film to seal the microplate as a whole and each well individually and prevent disturbance of the filtration process overall by collapse of the film into each well, at the particular filtration rate within each well, following the reduction in liquid level in the well, as the filtration process proceeds.